1. Field of the Invention
The present invention relates to an imaging unit which includes an imaging optical system and an image sensor.
2. Description of the Related Art
In recent years, mobile electronic devices which are designed mainly for taking still/moving photographic images, such as digital cameras (still-video cameras) and digital camcorders (motion-video cameras), and other mobile electronic devices which are designed to have the capability of taking such photographic images as a subsidiary function, such as mobile phones equipped with a camera and personal digital assistants (PDAs) equipped with a camera, have become widespread. In these types of mobile electronic devices, it is common to provide the mobile electronic device therein with an imaging unit which is structured such that an image sensor and a photographing optical system for leading light emanating from a photographic object (object-emanated light) to the image sensor are accommodated in a hollow housing of the imaging unit.
FIG. 6 shows an imaging unit of the related art.
This imaging unit 100 is provided with a hollow housing 101. A first lens element 102 is fixed to an end of the housing 101 on the object side (front side) in an exposed manner. An incident-side prism 103, a second lens group 104 and a third lens group 105 are fixed inside an internal space of the housing 101. The incident-side prism 103 is positioned immediately behind the first lens element 102 (the intersecting angle between the incident surface 103a and the exit surface 103c of the incident-side prism 103 is 90°), and the second lens group 104 faces the exit surface 103c of the incident-side prism 103. An exit-side prism 106 is fixed to the housing 101 in the internal space thereof at the opposite end of this internal space from the incident-side prism 103 (the intersecting angle between the incident surface 106a and the exit surface 106c is 90°). In addition, although not shown in FIG. 6, an optical element is installed between the third lens group 105 and the exit-side prism 106. An image sensor (image pickup device) 107 is fixed immediately in front of the exit-side prism 106. Additionally, the front of the housing 101 except an end portion of the front on the first lens element 102 side is entirely open, and this front opening is closed by a circuit board 108 to which the image sensor 107 is mounted. Additionally, a cover 109 covers this front opening of the housing 101 from the front to prevent the circuit board 108 from coming off the front opening of the housing 101.
Light rays, which are reflected by a photographic object (object to be photographed; not shown) positioned in front of the imaging unit 100, to be incident on the first lens element 102 pass through the first lens element 102 rearwardly (downwardly with respect to FIG. 6) to be incident on the incident surface 103a of the incident-side prism 103. Subsequently, the light rays incident on the incident surface 103a emerge from the exit surface 103c of the incident-side prism 103 after being reflected by a reflecting surface 103b of the incident-side prism 103 in an orthogonal direction (direction orthogonal to the optical axis of the first lens element 102). Subsequently, the light rays emerging from the exit surface 103c pass through the second lens group 104, the third lens group 105 and the aforementioned optical element (not shown) to be incident on the incident surface 106a of the exit-side prism 106. The light rays incident on the incident surface 106a are reflected forward by a reflecting surface 106b of the exit-side prism 106 to be captured (received) by the image sensor 107 (imaging surface thereof) after emerging from an exit surface 106c of the exit-side prism 106.
An example of the above-described related art (shown in FIG. 6) is disclosed in Japanese Unexamined Patent Publication No. 2008-242446. Other examples of bending optical systems are disclosed in Japanese Unexamined Patent Publication Nos. 2004-355010, 2007-6318 and 2009-116074.
Although the demand to slim down this type of imaging unit has become stronger, there is no choice but to use a large image sensor that has a large thickness as the image sensor 107 because the demand for an increased number of pixels (demand for higher resolution) in imaging units has also been very high. On the other hand, the first lens element 102 can be slimmed down while meeting the demand to increase the number of pixels of the image sensor 107 (demand for higher resolution) in imaging units by increasing the refractive power of the first lens element 102 or making an adjustment to the power of the first lens element 102. Therefore, if the position of the rear surface (exit surface) of the slimmed-down first lens element 102 and the position of the rear surface of the large image sensor (107) that has a large thickness in the forward/rearward direction are made to be substantially coincident with each other (if the position of the incident surface 103a of the incident-side prism 103 and the position of the exit surface 106c of the exit-side prism 106 in the forward/rearward direction are made to be substantially coincident with each other so as to substantially lie on a common plane), the cover plate 109 is positioned further forward (closer to the object side) relative to the front surface of the first lens element 102 (consequently forming a large projection and a large depression on the front of the imaging unit 100), which makes it impossible to slim down the entire imaging unit 100 even though the first lens element 102 is slimmed down.